#include <cstdio>
#define NMAX 100023
long double x[NMAX],y[NMAX];
int main()
{
    freopen ("aria.in","r",stdin);
    freopen ("aria.out","w",stdout);
    int n;
    long double a=0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        scanf("%Lf %Lf",&x[i],&y[i]);
        printf("%Lf %Lf\n",x[i],y[i]);
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(int i=1;i<=n;i++) a+=x[i]*(y[i+1]-y[i])-(x[i+1]-x[i])*y[i];
    printf("%.5Lf",a);
}